There are 6 ocular muscles responsible for normal eye movement. Historically when an eye removal (enucleation) was performed, the muscles were detached and not re-used. Glass, and then later silicone, spheres were used to fill the orbital void. These were simply placed into the orbit to replace the lost volume and did not transmit the movement generated from the disconnected ocular muscles, leading to an unnatural cosmetic effect. Furthermore, sagging of the lower eyelids occurred due to the high weight of the implants. The implants could later extrude, and could be uncomfortable.
In 1987 Perry A. C., (USA) described the use of a porous hydroxyapatite implant derived from natural coral as a substitute to the glass or silicone spheres. More recently in 1993, Porex Corp (USA) disclosed a similar porous product comprised of porous polyethylene. Several other products similar to the coral derivative now exist. FCI (CA) supply a porous implant of a similar nature comprised of synthetic alumina, and another called Molteno M-Sphere is derived from bovine cancellous bone. The porous nature of the device acts as a scaffold for the soft orbital tissue, including muscle tissue, that is in contact with the sphere and as such allows the tissue to grow entirely throughout the porous structure until it completely infiltrates all the pores creating a bicontinuous composite of porous implant and vascularised orbital tissue causing it to become a permanently integrated fixture. The ingrowth of the eye muscles anchors the muscles to the implant which then also transmits movement to the implant in conjunction with the remaining eye. However, there is in some cases the need for revision surgery to correct misalignment of eye movement using a peg system partly because the re-attachment points of the muscles are at the discretion of the operating surgeon.
In order to reattach the muscles they must be sutured in such a way so that they are in contact with the porous implant in the correct locations. Porous polyethylene, being a soft polymer, has the advantage that the muscles can be sutured directly to the implant. However the general opinion is that the biocompatibility of porous polyethylene is inferior to that of porous bioceramic implants, which are preferred by many surgeons in order to reduce complications to a minimum. Direct tissue apposition to the implant surface does not occur, instead a fibrous tissue layer first covers the implant.
Porous bioceramic implants are, like all ceramic materials, hard and naturally brittle and it is impossible, when attaching the muscles, to suture directly to the implant. When implanted, only the anterior-most portion is accessible and even if small tunnels could be created just through the anterior hemisphere through which suture needles could pass, it would be extremely difficult to carry this out during surgery and highly likely to damage the implant in the process. To overcome this problem, the surgeon can attempt to suture the muscles over and across the implant rather than directly to it, but the result is unreliable in terms of muscle reattachment and position. Alternatively, and far more commonly, a resorbable fabric mesh is wrapped entirely around the implant and secured at the posterior to create a tight mesh sac around the implant. The muscles can then be sutured directly to the fabric mesh.
The current technology however, presents a number of problems.
The direct insertion of porous ceramic implants can be very problematic as the sharp spicules of the broken pores (See FIG. 1.(1)) at the rough surfaces of the implants exhibit a dragging effect on the orbital tissue, grabbing the tissues and inhibiting proper deep seating of the implant within the orbital cavity. Mesh wrapped implants do not alleviate this problem as the mesh itself is insufficiently smooth.
Additionally, the rough surfaces can increase the chance of exposure of the implant because of its rubbing on the wound closure.
Other wrapping materials have been used which are smooth such as sclera, bovine pericardium and temporalis fascia to which the muscles can also be sutured. These wrapping materials are smooth and do allow proper insertion with ease. However, windows and holes must be cut into the surfaces to allow the soft tissue and the ocular muscles access to the macroporous inner. Secondary, the use of donor sclera and bovine pericardium is now discouraged due to the possibility of infection and a self graft of temporalis fascia is an additional surgical step and site, and not used unless necessary.
A number of problems exist with porous orbital implant technology:
a) suturing the muscles directly to a ceramic implant surface
b) suturing the muscles in the correct location
c) the problem of creating a smooth ceramic surface to allow deep placement within the orbit and minimize implant exposure rates
d) the said smooth surface must immediately allow the unrestricted ingrowth of soft tissue in order to minimize healing time
e) the device construction materials must have excellent biocompatibility
f) the implant must be lightweight
g) the macropores in the body of the implant must have complete interconnectivity to avoid the possibility of infection
The difficulties are interrelated, for example it would be of no practical use to create a smooth-surfaced implant unless the problem of suturing the muscles directly to the implant is first addressed, as a second material would still be required around the surface.
Although current devices have addressed some of these problems individually, there is a definite need for a spherical ceramic device which would address all of them. It would be advantageous for a device to provide a method of guided muscle re-attachment, eliminating both surgical error and the need to wrap the implant. If such a device could be provided, it would be more advantageous if the implant were to additionally have a smooth surface allowing the implant to be placed directly and easily, deep within the orbit, helping to prevent unwanted post-operative exposure. It would be of even greater benefit if the smooth surface were macroporous, allowing the maximum unrestricted ingrowth of vascular tissue without the need to have windows or holes cut through it. Finally it would be necessary for the implant body to be composed of a highly biocompatible bioceramic material and have a lightweight macroporous inner body that is completely interconnected and of a pore size suitable for the ingrowth and support of vascular tissue